Equity of American Assets Trust, Inc. Computation of Basic and Diluted EPS (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
NUMERATOR
Income from continuing operations									$ 40,132	$ 45,637	$ 53,915
Less: Net income attributable to restricted shares	$ (60)	$ (60)	$ (61)	$ (60)	$ (61)	$ (42)	$ (43)	$ (43)	(241)	(189)	(168)
Less: Income from operations attributable to unitholders in the Operating Partnership									10,814	12,863	15,238
NET INCOME ATTRIBUTABLE TO AMERICAN ASSETS TRUST, INC. STOCKHOLDERS	$ 7,077	$ 9,094	$ 5,519	$ 7,387	$ 8,861	$ 8,474	$ 7,599	$ 7,651	29,077	32,585	38,509
Income from operations attributable to American Assets Trust, Inc. common stockholders—basic									29,077	32,585	38,509
Plus: Income from operations attributable to unitholders in the Operating Partnership									(10,814)	(12,863)	(15,238)
Net income attributable to common stockholders-diluted									$ 39,891	$ 45,448	$ 53,747
DENOMINATOR
Weighted average shares of common stock outstanding-basic (in shares)									46,715,520	45,332,471	44,439,112
Effect of dilutive securities-conversion of Operating Partnership units (in shares)									17,371,730	17,895,688	17,900,051
Weighted average common shares outstanding - diluted (in shares)									64,087,250	63,228,159	62,339,163
Earnings (loss) per common share-basic
Basic net income attributable to common stockholders per share (in dollars per share)									$ 0.62	$ 0.72	$ 0.87
Earnings (loss) per common share-diluted
Diluted net income attributable to common stockholders per share (in dollars per share)									$ 0.62	$ 0.72	$ 0.86